Internal and External Zones on One Server

I have DNS configured with internal and external AD zones on the single DC server: int.corp.com and corp.com. Everything functions fine, but the internal namespace can be queried externally.

I know I can solve this by splitting the zones onto different servers and controlling external/internal lookups by controlling to where incoming external DNS traffic is routed.

Short of that, is there anything else that can be done to solve this and hide the internal namespace from external lookups?

Thanks!

Philip Heath